1. Field of Invention
The invention relates generally to drill bits for drilling boreholes in subsurface formations. More particularly, the present invention relates to designing drill bits, evaluating cutting structures, and designing cutting elements in view of the evaluating of the cutting structure.
2. Background Art
FIG. 1 shows one example of a conventional drilling system used in the oil and gas industry for drilling wells in earth formations. The drilling system includes a drilling rig (10) used to turn a drill string (12), which extends downward into a well bore (14). Connected to the end of the drill string (12) is a drill bit (20). The drill bit (20) is designed to break up and gouge earth formations (16) when rotated on the formations (16) under an applied force. Formation (16) broken up by the drill bit (20) during drilling is removed from the well bore (14) by drilling fluid typically pumped through the drill string (12) and drill bit (20) and up the annulus between the drill string (12) and the well bore (14).
One example of a conventional drill bit is shown in FIG. 2. This type of drill bit is typically referred to as a roller cone drill bit. A roller cone drill bit (20) includes a bit body (22) having a threaded section (24) at its upper end for securing to the drill string (12 in FIG. 1) and a plurality of legs (25) extending downwardly at its lower end. A frusto-conical rolling cone cutter (hereafter referred to as roller cone 26) is rotatably mounted on each leg (25) by a bearing shaft pin, which extends downwardly and inwardly from each leg (25). Each of the roller cones (26) has a cutting structure comprising a plurality of cutting elements (28) arranged on the conical surface of the cones (26). The cutting elements (28) project from the cone body and act to break up earth formations at the bottom of the borehole when the bit (20) is rotated under an applied axial load. The cutting elements (28) may comprise teeth formed on the conical surface of the cone (26) (typically referred to as milled teeth) or inserts press-fitted into holes in the conical surface of the cone (26) (such as tungsten carbide inserts).
Many prior art roller cone drill bits have been found to provide poor drilling performance due to problems such as “tracking” and “slipping.” Tracking occurs when cutting elements on a drill bit fall into previous impressions formed in the formation by cutting elements at a preceding moment in time during revolution of the drill bit. Slipping is related to tracking and occurs when cutting elements strike a portion of previous impressions and slides into the previous impressions.
In the case of roller cone drill bits, the cones of the bit typically do not exhibit true rolling during drilling due to action on the bottom of the borehole (hereafter referred to as “the bottomhole”), such as slipping. Because cutting elements do not cut effectively when they fall or slide into previous impressions made by other cutting elements, tracking and slipping should be avoided. In particular, tracking is inefficient since there is no fresh rock cut, and thus constitutes a waste of energy. Ideally, every contact of a cutting element on a bottomhole cuts fresh rock. Additionally, slipping should also be avoided because it can result in uneven wear on the cutting elements, which can result in premature failure.
In prior art bits, preventing premature failure due to tracking and slipping is typically accomplished by increasing the hardness of the cutting inserts. For example, U.S. Pat. No. 4,940,099 discloses a rotary drill bit having a plurality of cutters (i.e., roller cones) with rows of cutting inserts. Particularly, certain cutting inserts in a row have cutting surfaces formed with a wear-resistant material having a hardness higher than the hardness of a wear-resistant material on the remaining cutting inserts in the row. In this case, the cutting inserts are positioned in a predetermined pattern intermingled in a generally uniformly spaced pattern with the softer cutting inserts.
However, it has been found that tracking and slipping often occur due to a less than optimum spacing of cutting elements on the bit. Typically, the less than optimum spacing of cutting elements is a generally uniform spaced pattern. In many cases, by making proper adjustments to the arrangement of cutting elements on a bit, problems such as tracking and slipping can be significantly reduced. This is especially true for cutting elements on a drive row of a cone on a roller cone drill bit because the drive row is the row that generally governs the rotation speed of the cones.
Currently, cutting arrangements, such as the arrangement of cutting elements on rows of a roller cone drill bit are designed either by “gut feel,” in reaction to field performance, such as the addition of odd pitches to alleviate tracking and slipping, or by trial and error in conjunction with other programs used to predict drilling performance. The problem in these design approaches is that the resulting arrangements are often arrived at somewhat arbitrarily, which can be time consuming in the evolution of the bit design and may or may not lead to drill bits producing desired drilling characteristics.
Therefore, methods for predicting drilling characteristics prior to the manufacturing of drill bits are desired to reduce costs associated with designing bits and to enhance the development of longer lasting bits and/or bits which more aggressively drill through earth formations. Methods are also desired to minimize or eliminate the design and manufacturing of ineffective drill bits which exhibit significant tracking or slipping problems during drilling. Methods are also desired to reduce the time required for designing effective drill bits. Additionally, drill bit designs that exhibit reduced tracking and slipping over prior art bit designs are also desired.